The confined variational method is used to generate a set of energy-optimized explicitly correlated Gaussians to describe positrons interacting with H, He and the H2 molecule. These basis functions are then used in a Kohn variational calculation of the scattering lengths and zero-energy annihilation parameters, Zeff. The results for the H and He atoms are consistent with previous high-quality variational calculations while the annihilation parameter for H2 is consistent with experiment. An extensive tabulation of the H2 scattering parameters as a function of internuclear distance confirm the existence of a virtual state at R≈3.4a0. The scattering length was -2.71a0 and Zeff was 15.7 at the mean H2 internuclear distance of 1.45a0.